1. Field
The present invention relates to a calibration method to calibrate an encoder position measurement system of a stage, to a stage system and to a lithographic apparatus comprising such stage system.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, use may be made of an encoder system to measure a position of the stage. Thereto, a grid plate may be provided which is mounted to a reference structure of the lithographic apparatus. Measurements may be taken by sensor heads which are connected to the stage. Use may be made of sensor heads which show a sensitivity in two dimensions, e.g. by interaction of measurement beam having an oblique incidence angle with the grating, thereby providing a single output signal which exhibits a sensitivity to a movement of the stage in both dimensions. By suitably combining (e.g. adding, subtracting) etc. output signals of two of such sensor heads, information can be obtained as to a displacement in the one dimension as well as in the other dimension.